This project was developed and is directed by C.H. Paik, Ph.D. The research has centered on improving the tumor-targeting property of monoclonal antibodies and the fragments by chemical modifications. For FY 2000 we have investigated the use of a novel multistep tumor targeting. Our strategy involves pretargeting tumors with monoclonal antibody (MoAb) peptide followed by the injection of a radiolabeled second peptide. This approach decouples the antibody injection from the radiolabel injection, thereby offering the specificity of antibody binding to tumor antigens while eliminating problems associated with slow blood clearance of radiolabeled MoAb due to its large size. This project involves the synthesis of MoAb-peptide conjugates, clearing agents, and radiolabeled peptides. We have been optimizing the synthesis of these reagents to maintain the integrity of the immunoreactivity of MoAb and allow dimerization formation of peptides. Our preliminary multistep tumor-targeting experiments using tumor-bearing nude mice showed high accumulation of a radiolabeled peptide in tumor tissues and low accumulation in non-tumor tissues and blood, thereby providing high tumor-to-background radioactivity ratios.